Days before returning home from a whirlwind trip to Asia Donald Trump has gone back on the defensive over Russian election meddling and taunted the leader of North Korea.

The US President said he considers President Vladimir Putin's denials sincere, dismissed former US intelligence officials as "political hacks" and accused Democrats of trying to sabotage relations between the two countries.

Speaking to reporters aboard Air Force One, Mr Trump said Mr Putin had again vehemently insisted - this time on the sidelines of an economic summit in Vietnam - that Moscow had not interfered in the 2016 US elections.

Mr Trump declined to say whether he believed Mr Putin, but he made clear he wasn't interested in dwelling on the issue.

"He said he absolutely did not meddle in our election. He did not do what they are saying he did," Mr Trump said as he travelled to Hanoi, the second-to-last stop of his Asia trip.

"Every time he sees me, he said: 'I didn't do that.' And I believe - I really believe - that when he tells me that, he means it," Mr Trump said.

He called the accusation an "artificial barrier" erected by Democrats - once again casting doubt on the US intelligence community's conclusion that Russia tried to interfere in the election to help the Republican Trump beat Democrat Hillary Clinton.

In a tweet sent on Sunday from Hanoi, Mr Trump bashed the "haters and fools" he said are questioning his efforts to improve relations with Russia and accused critics of "playing politics" and hurting the country.

He also exchanged playground taunts with North Korea's Kim Jong Un.

Mr Trump said in a tweet from Vietnam: "Why would Kim Jong Un insult me by calling me "old," when I would NEVER call him "short and fat?""

Mr Trump added sarcastically, "Oh well, I try so hard to be his friend" and "maybe someday that will happen!"

The President had been working to rally global pressure against North Korea's nuclear weapons program on the trip to Asia, including a stern speech delivered in South Korea.

Mr Kim's government responded to that speech by calling Mr Trump an "old lunatic".

Mr Trump and Mr Putin did not have a formal meeting while they were in Vietnam for the Asia-Pacific Economic Cooperation (Apec) summit, but the two spoke informally several times and reached an agreement on a number of principles for the future of war-torn Syria.

Mr Trump made clear that the issue of Russian meddling in the election hovers over the leaders' relationship and said it jeopardised their ability to work together on issues including North Korea's escalating nuclear program and the deadly conflict in Syria.

"Having a good relationship with Russia's a great, great thing. And this artificial Democratic hit job gets in the way," Mr Trump told reporters. "People will die because of it."

Mr Trump danced around the question of whether he believed Putin's denials, telling reporters that pressing the issue would have accomplished little.

"Well, look, I can't stand there and argue with him," Mr Trump said. "I'd rather have him get out of Syria, to be honest with you. I'd rather have him, you know, work with him on the Ukraine than standing and arguing about whether or not - 'cause that whole thing was set up by the Democrats."

Multiple US intelligence agencies have concluded that Moscow meddled in the 2016 election to try to help Mr Trump win. But Mr Trump called the former heads of those agencies "political hacks" and argued there's plenty of reason to be suspicious of their findings.

The comments made clear that Mr Trump still does not take the meddling seriously and sees little benefit in punishing a nation accused of undermining the most fundamental tenet of American democracy: free and fair elections.

They also suggest that Mr Trump is unlikely to work aggressively to try to prevent future meddling despite repeated warnings from senior intelligence officials that Russia is likely to try to interfere again.

Meanwhile, a special counsel investigation of potential collusion between Moscow and Mr Trump's campaign aides so far has resulted in two indictments for financial and other crimes unrelated to the campaign, as well as a guilty plea. Congressional committees have also been interviewing campaign and White House staff.